This invention relates, in general, to amplifiers, and more particularly, to an amplifier having a linear transfer function.
An amplifier is a circuit that receives an input signal and provides an amplified output signal. The input signal to output signal transfer function is often a critical parameter when choosing an amplifier for a specific application. In particular, an amplifier may be required to have a linear input signal to output signal transfer function. Non-linearities in the transfer function of a linear amplifier will produce distortion components of the input signal.
An example of a circuit using an amplifier having a linear transfer function is a mixer circuit for mixing a high frequency carrier signal with a local oscillator signal. The mixer circuit typically reduces the frequency of the high frequency carrier signal. Characteristics used to measure the performance of a mixer circuit are gain, noise, quiescent current, peak available current, linearity, and distortion. Linearity is of special importance because it determines the accuracy in which the lower frequency is generated. Any non-linearities in the mixer circuit may also generate intermodulation frequencies that affect circuit performance.
A common circuit used as a linear amplifier is a differential input stage (a differential pair of transistors) biased by a current source. Degeneration resistors are used to linearize the response of a differential input stage when non-linear element such as bipolar transistors are used in the differential input stage. A degeneration resistor dominates the non-linear characteristics of a bipolar transistor producing a linear response. The linearity can be increased by increasing the degeneration resistor but comes at the expense of reducing transconductance and increasing noise. In general, an enhancement of specific performance characteristics usually results in a degradation of other operating parameters.
It would be of great benefit if an amplifier could be provided that provides a linear transfer function without compromising other performance factors.